The main objective of the present thesis is to investigate the mechanisms underlying the age- and size-related decline observed in growth efficiency and in relative above ground mass growth rates in both Scots pine and poplar (clonal) chronosequences. Hence, I compared differentially aged Scots pine stands and trees growing at the same site, in contrast to most previous studies, isolating leaf- from stand-level responses. In addition, I used poplar clonal material to compare growth and morphological parameters of genetically identical cuttings taken from young, middle-age and old plants belonging to four clones and I carried out investigations both in the field on the original donor trees (trees of identical meristematic ages and different sizes) and on rooted cuttings (identical meristematic ages and same size). Scots pine study: Our results partially supported the hydraulic limitation hypothesis, but at the same time suggested that additional factors were also involved in the decline in growth efficiency with age. Such factors may involve decreased soil nutrient availability, increased below-ground allocation and reduced turgor pressure in tall trees. Poplar study: The observed decline in growth and carbon uptake in poplar donor trees in the field was related to the increases in tree size, the increased path length and possibly the architectural complexity, or in some cases to decreased nutrient availability in the soil but not to cellular senescence per se.